Dense wavelength multiplexing is a technique for producing a single, high-brightness, multi-spectral, combined output beam from a plurality of individual, narrow spectral bandwidth input beams. DWM enables multiple relatively low-power single wavelength input beams to be superimposed to produce a single, high-power, high-brightness output beam. DWM techniques enable output beam power to be scaled directly with the sum of the power produced by the plurality of input beams and to further provide output beams with beam quality commensurable with the beam quality of the individual input beams.
In DWM systems, a plurality of narrow spectral bandwidth, or single wavelength, laser beams are emitted from a laser source that comprises a plurality of individual emitters. The multi-spectral output beam is formed by combining, or spatially and directionally overlapping, the plurality of individual beams with a beam overlapping element. Beam combining can be achieved through selecting a single wavelength for each of the beams and directing each of the beams at the beam overlapping element with a particular angle of incidence. The wavelength and angle of incidence of each beam is selected such that all of the beams emerge from the beam overlapping element at an overlap region with a common direction of propagation. A set of allowed wavelength-angle pairs can be defined as all combinations of wavelength and angle of incidence that will yield a beam that emerges from the beam overlapping element at the common direction of propagation.
In order to produce a single multi-spectral combined output beam from the plurality of laser beams emitted by the laser source, a wavelength-angle pair from the set of allowed wavelength-angle pairs must be selected for each emitter in the laser source. Angle of incidence selection can be accomplished by fixing the relative position of the laser source and beam overlapping element and placing a position-to-angle transformation lens at a fixed position in the optical path between the laser source and the beam overlapping element. The position-to-angle transformation lens maps the spatial position of each emitter in the laser source to a particular angle of incidence at the beam overlapping element.
For each individual emitter, wavelength selection can be accomplished by providing feedback to the emitter in the form of electromagnetic radiation with the desired wavelength. Providing such electromagnetic radiation to the emitter will excite a resonant mode of the emitter corresponding to the desired output. Thus, providing feedback to the emitter will stimulate the emission of additional electromagnetic radiation with a wavelength that is equivalent to that of the feedback. The resonant feedback will narrow the spectral bandwidth of the laser beam emitted by the emitter and center the wavelength spectrum of the emitted beam about the wavelength of the resonant feedback. This process of providing feedback to an emitter can be referred to as beam wavelength stabilization, or wavelength locking.
Locking the wavelength of each laser beam maps a single wavelength to each position of an emitter in the laser source and creates a set of fixed wavelength-position pairs for the laser source. The position-to-angle transformation lens maps the wavelength-position pair for each emitter in the laser source to a particular wavelength-angle pair. Selecting appropriate wavelength-position pairs ensures that a spatially and directionally overlapped beam will be produced. However, if the wavelength locking is not robust and alternative resonant modes of the individual emitters are excited, the emitters will produce alternative resonant mode components that will thereafter propagate through the system. The beam components produced by the alternative resonant modes do not represent allowed wavelength-position pairs and will therefore not be spatially and directionally overlapped by the beam overlapping element. Furthermore, if such alternative resonant modes are allowed to propagate through an external resonator that provides feedback to the laser source in order to stabilize the wavelength of the beams emitted by the emitters in the laser source, these alternative resonant modes will stimulate the emission of further parasitic, alternative mode components and thereby degrade output beam quality and induce temporal fluctuations in output beam power.